SITE-SPECIFIC AND LANDSCAPE FEATURES ASSOCIATED

WITH SHRUBLAND BIRD OCCURENCE IN ANTHROPOGENIC

SHRUBLANDS IN THE NORTHEASTERN UNITED STATES

BY

RANDY SHOE

Wildlife Conservation and Biology BS, University of New Hampshire, 2015

THESIS

Submitted to the University of New Hampshire

in Partial Fulfillment of

the Requirements for the Degree of

Master of Science

in

Natural Resources:

Wildlife and Conservation Biology

December 2018

ii

This thesis has been examined and approved in partial fulfillment of the requirements for the

degree of Master of Science in Natural Resources: Wildlife and Conservation Biology by:

Thesis Director, John Litvaitis, Ph.D., Professor Emeritus

Thomas Lee, Ph.D., Associate Professor, Natural Resources & The Environment

Matthew Tarr, Ph.D., Extension Professor, UNH Cooperative Extension

Mariko Yamasaki, U.S. Forest Service, Research Wildlife Biologist

Don Keirstead, Natural Resources Conservation Service, State Resource Conservationist

On 29 November 2018

Original approval signatures are on file with the University of New Hampshire Graduate School.

iii

ACKNOWLEDGEMENTS

I would like to thank the Nuttall Ornithological Club, the Natural Resources Conservation

Service, the New Hampshire Fish and Game Department, and the University of New Hampshire

for their generous financial support. I would like to acknowledge my committee and the

following people who made this possible: Jack and Kamron Hernandez, Erin Smith, Kyle Crafts,

Jennifer Gibson, Ryan Brown, Elizabeth Godin, Mike Doherty, Casey Coupe, Cooper Bryer,

Brittany Taylor, Rory Carroll, Avery Shoe-Ricker, Amir Kirata, Beau Garcia, and Shane Bradt.

Some special thanks go to my right-hand support and friend, Erica Holm. I would like to express

my thanks and love to my daughters, Stacy and Stefanie, and my wife Shirley for their support

and love during this project; for my success would be nothing without them.

iv

LIST OF FIGURES

FIGURE 1. Distribution of 101 anthropogenic shrublands surveyed for shrubland birds in

southeastern New Hampshire, 2015-2016.

FIGURE 2. Comparing the likelihood that each of eight focal shrubland bird species was

detected within each of four anthropogenic shrubland types occurring in Rockingham and

Strafford Counties, New Hampshire May-August 2015 and 2016.

FIGURE 3. Best NPMR models predicting focal bird occurrence and bird sensitivity to an

increasing proportion of site-specific and surrounding landscape features.

FIGURE 4. The proportion of the site-specific features (SSF) and surrounding landscape

features (SLF) comprising the best fit NPMR models predicting focal species occurrence in 101

anthropogenic shrublands in southeastern New Hampshire 2015-2016.

FIGURE 5. Average percent change in predicted occurrence for eight focal shrubland bird

species in response to increasing proportions of Shrubland, Field/Pasture, and Urban

Development in the surrounding landscape (50 m – 10 km) from the best fit NPMR models.

FIGURE 6. Nonmetric multidimensional scaling (NMDS) of eight focal shrubland bird species

in a community structure of 101 anthropogenic shrublands in southeastern New Hampshire,

USA, 2015-2016.

FIGURE 7. NPMR comparing predicted occurrence of eight focal shrubland bird species in

response to increasing Vegetation Density estimated in 101 anthropogenic shrublands in

Strafford and Rockingham Counties New Hampshire 2015-2016.

FIGURE 8. NPMR comparing predicted occurrence of eight focal shrubland bird species in

response to increasing Shrubland Size (ha) estimated in 101 anthropogenic shrublands in

Strafford and Rockingham Counties New Hampshire 2015-2016.

TABLE 1. LANDFIRE 2014 Existing Vegetation Height (30 m pixels) categories grouped into

seven surrounding landscape features with reasons for grouping.

v

LIST OF TABLES

TABLE 1. LANDFIRE 2014 Existing Vegetation Height (30 m pixels) categories grouped into

seven surrounding landscape features with reasons for grouping.

vi

ABSTRACT

SITE-SPECIFIC AND LANDSCAPE FEATURES ASSOCIATED WITH SHRUBLAND BIRD

OCCURRENCE IN ANTHROPOGENIC SHRUBLANDS IN THE NORTHEASTERN

UNITED STATES

By

Randy Shoe

University of New Hampshire, December 2018

Habitats dominated by low-growing trees and shrubs are becoming increasingly uncommon in

the northeastern U.S. Human development, altered natural-disturbance regimes, and forest

succession have reduced the quantity and quality of these shrublands. As a result, over half of the

shrubland-dependent songbirds in the region have experienced long-term population declines.

Anthropogenic shrublands, including regenerating clearcuts, sand and gravel mines, old fields,

and transmission line rights-of-way may provide nesting habitat for most shrubland birds; but

differences in size, site-specific features, and landscape composition may affect bird use. To

assess the features that may influence shrubland bird occurrence in anthropogenic shrublands, I

conducted presence/absence surveys of 8 species [alder flycatcher (Empidonax alnorum), brown

thrasher (Toxostoma rufum), blue-winged warbler (Vermivora cyanoptera), chestnut-sided

warbler (Setophaga pensylvanica), eastern towhee (Pipilo erythrophthalmus), field sparrow

(Spizella pusilla), indigo bunting (Passerina cyanea), and prairie warbler (Setophaga discolor)]

in 101 sites in southeastern New Hampshire during the 2015 and 2016 nesting seasons. For each

shrubland, I measured area, site-specific features (e.g., vegetation height, density, and coverage),

and characteristics of surrounding landscape features within different buffer zones. Overall, 67%

of the variables in the best models predicting bird occurrence were landscape features and 33%

were site-specific features. Bird occurrence at a site was positively associated with the proportion

vii

of shrublands in the surrounding landscape, particularly within a 500 m buffer. Occurrence of all

species except blue-winged warblers and indigo buntings was negatively associated with the

proportion of urban development in the surrounding landscape. Shrubland bird species richness

increased with vegetation density until vegetation density became too dense for brown thrashers,

field sparrows, and prairie warblers. Occurrence of all species except blue-winged warblers

increased with shrubland size. These results provide opportunities to enhance existing

anthropogenic habitats to benefit populations of declining shrubland birds.

viii

TABLE OF CONTENTS

ACKNOWLEDGEMENTS………………………………………………….. iii

LIST OF FIGURES………………….……….………………………………. iv

LIST OF TABLES…………………………………………………...……….. v

ABSTRACT………………………………………………………………….. vi

CHAPTER PAGE

1. INTRODUCTION…………………………………………………….…….. 1

2. METHODS…………………………………………………………….……. 6

2.1. Study Area and Shrubland Selection……………………..………….…. 6

2.2. Focal Species……………………………………………………….…… 8

2.3. Field Methods………………………………………………….……….. 9

2.3.1. Site-Specific Features ………………………….……….……….. 9

2.3.2. Landscape Features ………………….………….……….………. 11

2.4. Statistical Analysis……………………………………………………… 13

2.4.1. Bird Occurrence …………………………………………….…… 13

2.4.2. Site-specific and Landscape Features Affecting Occurrence …… 13

3. RESULTS…………………………………………………………………… 15

3.1. Shrubland Bird Detection and Shrubland Types …………….………… 15

3.2. Bird Responses to Site-Specific and Landscape Features …………...… 16

3.3. Bird Reponses to Shrubland Size ……………………………………… 23

4. DISCUSSION……………………………………………………………….. 24

4.1. Importance of Landscape Composition to Breeding Shrubland Birds ……. 25

4.2. Vegetation Structure, not Species Composition, Predicted Shrubland Bird

Occurrence. …………………………………………………………….. 28

4.3. Shrubland Birds Occur More Predictably in Larger Shrublands ……...…... 32

5. CONCLUSION AND MANAGEMENT RECOMMENDATIONS ………. 33

LITERATURE CITED………………………………………….……………… 34

ix

APPENDIX A. Shrubland site-specific features, ranges and averages for sand

and gravel mines (S&GM), transmission line rights-of-way

(ROW), old fields (OF), and clearcuts (CC)…………………… 41

APPENDIX B. Shrubland surrounding landscape features, ranges and averages

for sand and gravel mines (S&GM), transmission line

rights-of-way (ROW), old fields (OF), and clearcuts (CC)……. 42

APPENDIX C. UNH Institutional Animal Care and Use Committee approval

letter …………………………………………………………… 43

1

1. INTRODUCTION

Habitats dominated by low-growing trees and shrubs are becoming increasingly uncommon in

the northeastern United States (Hunter et al. 2001, Motzkin and Foster 2002) where human

developments, altered disturbance regimes, and forest succession have reduced the quantity and

quality of shrublands that are important to many wildlife species (Litvaitis 1993, Askins 2001,

Thompson and DeGraaf 2001). As a result, shrubland-dependent wildlife, including 29 of 41

shrubland-dependent birds have experienced long-term population declines (Rosenberg et al.

2016). In response to these declines, federal and state natural resource agencies, in partnership

with public and private landowners, are working to create and maintain shrublands (Oehler et al.

2006). The largest and most recent of these efforts in New England and eastern New York

involves clearcutting large (> 5 ha) blocks of second growth forest near existing anthropogenic

shrublands such as sand and gravel mines, transmission line rights-of-way (ROW), and shrubby

old fields, to restore populations of New England cottontails (Sylvilagus transitionalis) (Fuller

and Tur 2012). While this effort is focused on establishing a network of shrublands that support

cottontail breeding and dispersal, these habitats and their arrangement are expected to provide

similar benefits to declining shrubland birds.

Throughout New England, populations of many shrubland bird species, including prairie

warblers (Setophaga discolor), blue-winged warblers (Vermivora cyanoptera), chestnut-sided

warblers (Setophaga pensylvanica), brown thrashers (Toxostoma, rufum), eastern towhees

(Pipilo erythrophthalmus), field sparrows (Spizella pusilla), and indigo buntings (Passerina

cyanea) rely on anthropogenic shrublands as their primary breeding habitat wherever natural

xeric shrublands such as pine barrens are absent (Dettmers 2003, Gifford et al. 2010). Here,

shrubby old fields, shrubby ROWs, active and idle sand and gravel mines, and regenerating

2

clearcuts are the most common anthropogenic habitats these birds use for breeding (Confer and

Pascoe 2003, Schlossberg and King 2007, King et al. 2009a). Although there is overlap in the

bird species expected to use each of these shrubland types (Rodewald and Vitz 2005, Bulluck

and Buehler 2006, Schlossberg and King 2007), differences in growing conditions, vegetation

structure, and plant species composition may lead to distinct bird communities among shrublands

(Skowno and Bond 2003, Bulluck and Buehler 2006, Schlossberg and King 2007). Further, the

extent to which shrubland size and shape, and landscape composition surrounding such

shrublands interact to influence habitat selection by shrubland birds has received limited study

(Askins et al. 2007, Chandler et al. 2009a, Roberts and King 2017). Understanding how these

factors influence habitat selection by shrubland birds is critical for determining the amount of

habitat currently available for these species and for guiding habitat management and

conservation efforts aimed at maintaining functioning bird populations.

Vegetation structure can have an important influence on bird community assemblages (Karr

and Roth 1971) and shrubland birds are recognized as habitat specialists that require or prefer

specific vegetation structure or composition for nesting and feeding (Schlossberg and King 2007,

King et al. 2009b, Schlossberg et al. 2010). As a result, differences in vegetation structure and

composition between anthropogenic shrublands may result in each bird species occurring in

different abundances among different shrubland types or being completely absent from

shrublands lacking required conditions (DeGraaf and Yamasaki 2003, Schlossberg and King

2007). Composition of bird communities in these habitats should also change as vegetation

structure changes (Schlossberg 2009). For example, by 10-15 years after cutting a stand of

northern hardwoods, woody vegetation has typically grown beyond the shrub stage required by

nesting shrubland birds and management is required to return the site to a shrubby condition

3

(DeGraaf and Yamasaki 2003, Dettmers 2003). Further, many ROWs in New England are

maintained in a shrubby condition by mowing with a brontosaurus-style forestry mower (a large

flailing-head mower attached to an excavator used to grind up shrubs and young trees) every 3-4

years to cut tree saplings that will grow tall enough to intercept the transmission lines (K.

Nelson, Eversource, personal communication). Immediately following mowing, the vegetation

along a ROW is typically dense herbaceous cover interspersed with low, scattered shrubs that

quickly succeed to very tall and dense shrubs and tree saplings during the 3-4 years between

mowing events. This succession of vegetation can result in wide variation in the composition and

density of the shrubland bird community within a ROW throughout a mowing cycle (Kroodsma

1982, King and Byers 2002, Askins et al. 2012). Understanding how differences in shrubland

vegetation structure influences the occurrence of different shrubland bird species is an important

step toward determining how different anthropogenic shrublands function as habitat.

Although vegetation structure is generally considered more important than plant species

composition when birds select habitats (MacArthur et al. 1962, James 1971, Karr and Roth

1971), shrub species composition may influence habitat use and selection by shrubland birds

(Willson 1974, Rotenberry 1985, Schlossberg et al. 2010). For example, preference for nesting in

non-native invasive shrubs vs. native shrubs varies among areas (Stoleson and Finch 2001,

Heckscher 2004, Schlossberg and King 2010). Differences in food resources, particularly insects

(Burghardt et al. 2010, Pendleton et al. 2011, Fickenscher et al. 2014) and fruits (Howe and

Estabrook 1977, Herrera 1982, Smith et al. 2013) can also vary between habitats dominated by

non-native vs. native plants and contribute to differences in the bird community among habitats

(Burghardt et al. 2009, Tarr 2017, E. Holm, University of New Hampshire, unpublished data).

Understanding whether shrubland birds select for or avoid anthropogenic shrublands containing

4

non-native shrubs is important for understanding differences in bird distribution among

shrublands and for guiding habitat management decisions regarding if and when non-native

shrubs should be controlled to benefit birds.

Factors other than vegetation structure and composition may also have an important influence

on shrubland bird use of anthropogenic shrublands. Specifically, the size and shape of shrublands

may be especially important because most shrubland birds are considered to be area sensitive

(Rodewald and Vitz 2005, Lehnen and Rodewald 2009a, Shake et al. 2012) and absent from

openings below some minimum size (Taylor and Taylor 1979, Costello et al. 2000, Askins et al.

2007). For example, Askins et al. (2007) suggested a minimum shrubland size of 0.6 ha for

shrubland birds and Costello et al. (2000) found clearcuts < 0.8 ha were too small to support

most shrubland bird species in northern New Hampshire. Larger shrublands are likely to support

greater shrubland bird richness due, in part, to the greater diversity of microhabitats they contain

compared to smaller openings (Rudnicky and Hunter 1993), but shrubland openings < 1.1 ha can

provide habitat for some shrubland birds if those openings contain required microhabitats

(Roberts and King 2017). Similarly, wide ROWs tend to support a greater diversity of shrubland

birds than narrow ROWs (Anderson et al. 1977, Confer and Pascoe 2003, Askins et al. 2012).

This area sensitivity may also be the result of shrubland birds preferring to nest away from

habitat edges where predation risk can be greater than within shrubland interiors (Shake et al.

2011). As a result, long and narrow shrublands (i.e., ROWs) may support a lower diversity and

abundance of shrubland birds than large and comparatively wider openings such as clearcuts

(Anderson et al. 1977, Schlossberg and King 2007, Askins et al. 2012), so differences in

shrubland size and shape may influence shrubland bird occurrence among anthropogenic

shrubland types.

5

Finally, bird habitat selection may also be influenced by the composition of the landscape

surrounding a site (Hinsley et al. 1995, Askins et al. 2012). Few studies have investigated the

importance of landscape composition to predict shrubland bird occurrence (e.g., Askins et al.

2007, Chandler et al. 2009b, Shake et al. 2012) and the results of these studies seem equivocal.

For example, in landscapes composed predominately by forest, wetlands, or shrublands (i.e., not

human-developed landscapes), Hagan and Meehan (2002) found that landscape composition

within 1 km of shrublands explained the presence/absence of only 2 out of 20 bird species and

Chandler (2006) found shrubland bird occurrences in wildlife openings was better predicted by

microhabitat variables within openings than by the proportion of shrublands in the surrounding

landscape. These results may be due, in part, to shrubland birds evolving stronger associations to

specific vegetation conditions within shrublands rather than to the composition of the matrix

landscape that was historically composed predominantly of mature forest (Askins et al. 2007).

However, in predominately developed landscapes, the dominant landcover may be especially

important for predicting shrubland bird occurrence in remnant shrublands (Roberts and King

2017), with the importance of the surrounding landscape composition extending beyond 100 m

from study shrublands (Hostetler and Knowles-Yanez 2003). As suburban development

increases in many areas of New England, understanding how landscape habitat composition

influences shrubland bird distribution among shrublands is important for planning landscape-

level habitat management and conservation to benefit declining bird species.

To examine the role of site-specific features (e.g., vegetation composition and shrubland size)

and surrounding landscape composition, I examined the occurrence of common shrubland birds

in four types of anthropogenic shrublands; clearcuts, sand and gravel mines, old fields, and

ROWs. I predicted that site-specific features would have a greater influence on shrubland bird

6

occurrences than surrounding landscape features, that surrounding landscape features within 500

m of a shrubland would have a greater influence on bird occurrences than features farther away,

and that bird occurrences would be associated positively with the proportion of shrubland and

negatively with the proportion of urban development within the surrounding landscape.

2. METHODS

2.1. Study Area and Shrubland Selection

I conducted this study in seacoast of southeastern New Hampshire, in Rockingham and

Strafford counties (Fig 1). The study area (2873 km2) contains approximately 70% Forest, 14%

Urban Development, 7% Fields/Pastures (i.e., habitats composed predominantly of herbaceous

vegetation < 1m tall), 4% Open Water, 4% Shrubland (i.e., habitats composed of thickets of

shrubs and young trees mixed with scattered grasses and wildflowers), 0.2% Tidal Vegetation

(i.e., habitats composed of grasses and herbaceous vegetation influenced by salt water influx)

and 0.2% Agricultural (i.e., lands managed for crops). The dominant forest type in this area is

eastern hemlock (Tsuga canadensis) – American beech (Fagus grandifolia) - oak (Quercus spp.)

- white pine (Pinus strobus) forest (Sperduto and Nichols 2012). There is more urbanization,

open water, and tidal vegetation near the coast leading into a more forested landscape as the

distance increases inland. Fields/Pastures and Agricultural landscape features are scattered

throughout the landscape as soil conditions permit.

The seacoast area of New Hampshire has unique anthropogenic shrublands, such as sand and

gravel mines, clearcuts, and ROW due to geological conditions (Sperduto and Nichols 2012),

New England cottontail habitat management (Fuller and Tur 2012), and urbanization

respectively. Old fields are less abundant due to forest succession (Litvaitis 1993). A total of 101

7

shrublands were selected as study sites: 26 clearcuts, 25 sand and gravel mines, 25 old fields, and

25 ROWs. Each shrubland was ≥ 0.8 ha and at least 250 m from the next nearest shrubland

surveyed. All ROWs were > 50 m wide and characterized by extensive shrub and sapling

hardwood growth that varied in density and vertical structure depending on time since it was

mowed. All ROWs were mowed with a brontosaurus-style forestry mower within the previous 1-

4 years and all vegetation was < 5 m tall. Study sites within ROWs varied from 1.1 to 26.5 ha

and each was considered a continuous shrubland until it was bisected by a road, parking lot,

housing development, agricultural field, or by a water feature that observers could not cross. All

silviculture openings (patch cuts, clearcuts, group selection cuts) were considered clearcuts and

were within 1-15 years of complete post-timber harvest and ranged from 1.1 to 23.7 ha. All

clearcuts had few standing trees, a hard forest/shrubland edge, and contained shrubs,

regenerating trees, and woody debris resulting in extensive vertical and horizontal diversity. Old

fields varied from 1.1 to 21.6 ha and were last mowed or grazed > 2 years prior to the study,

creating a variable vertical structure (0.2 to < 3 m tall) of dense grasses interspersed with shrubs

and small saplings. Sand and gravel mines varied in size from 1.7 to 90.9 ha and contained

extensive areas of overturned soils in a xeric environment, with pockets of shrubs, grasses and

saplings, and ponds of variable sizes created by mining activities. Active sand and gravel mines

(n = 11) were composed of a perimeter of shrubby habitat interspersed among areas where gravel

was being extracted. Idle sand and gravel mines (n = 14) were composed of a perimeter of taller

shrubs and small trees, and shrubby interiors interspersed with areas of bare ground.

8

FIGURE 1. Distribution of 101 anthropogenic shrublands surveyed for shrubland birds in

southeastern New Hampshire, 2015-2016.

2.2. Focal Species

Eight shrubland bird species were the focal species of this study: alder flycatchers, brown

thrashers, blue-winged warblers, chestnut-sided warblers, eastern towhees, field sparrows, indigo

buntings, and prairie warblers. These species are identified as “Stewardship Species of

Continental Importance” for the U.S. and Canada (Rosenberg et al. 2016), as “Species of

Greatest Conservation Need” in one or more New England state wildlife action plans, and they

require shrublands as their primary breeding habitat (Schlossberg and King 2007). Collectively,

these species require or prefer a range of shrubland habitat conditions (e.g., mesic, xeric, exposed

soils, dense/sparse shrub and tree cover, minimum required shrubland size) and they all have

experienced population declines in New England over the last 50 years (U.S. Fish and Wildlife

Service 2008, Rosenberg et al. 2016).

9

2.3. Field Methods

I conducted three presence/absence surveys for focal bird species in each shrubland between

May and August, with approximately half of the shrublands surveyed in 2015 and the other half

in 2016. Consecutive surveys at each shrubland were separated by ≥ 2 weeks. Each survey

consisted of a surveyor using a handheld speaker to broadcast the breeding songs of each focal

bird species throughout the entire shrubland to ensure complete coverage. Each focal species was

recorded as either present (detected by sight or sound) or absent (not detected) during each

survey. Once a species was detected, no further breeding song was broadcast for that species

during that survey. A species occurred at a site if it was present during at least one survey.

Surveys were conducted in the morning between sunrise and 1200 and they were not conducted

during rain, excessive wind (> 4 in Beaufort scale), fog (visibility < 200 m), or excessive heat

index (> 32 C).

2.3.1. Site-Specific Features

Eleven site-specific features were used to describe structural characteristics of sites

inventoried for breeding birds (Appendix A). Size and perimeter were determined for clearcuts,

sand and gravel mines, and old fields by walking the perimeter of the area with a handheld GPS

unit and calculating area in ArcGIS 10.2.2. I estimated size for ROWs from digital aerial

photographs in ArcGIS 10.2.2. Bare Ground Cover (the litter layer, rocks, and tree stumps), area

of Open Water, and the height, and percent cover of six vegetation classes (grasses, ferns, forbs,

native shrubs, non-native shrubs and trees) were estimated at 30 sampling points located

randomly within each shrubland. A mil-acre plot was established at each point using a 1.13 m

long rope attached to a pole held vertically at the center of the point, with the pole marked at 0.2

m intervals. The height of each vegetation class was estimated as the tallest specimen

10

representing each class (nearest 0.2 m, continuously from 0 to > 3 m). Any class missing or < 0.2

m tall was recorded as zero. Woody plants < 7.0 cm in diameter at breast height were considered

shrubs and those larger as trees. Visual estimates of the percent cover were made for bare

ground, water, and each vegetation class by looking directly down upon the plot. Cover estimates

were averaged from estimates collected simultaneously by ≥ 2 observers trained at the beginning

of each study season and weekly thereafter, to ensure consistent estimates among observers.

Because vegetation height correlated with percent cover for each vegetation class (r > 0.7), it was

not included as a model variable. Vegetation Density (all vegetation classes combined) in each

plot was measured with a density comparison gauge (5.1 cm x 7.6 cm x 10.2 cm down spout

adapter with 9 equidistant sections created by wire on the rectangular section). The density

comparison gauge was held directly against the eye 1 m above the ground and any portion of a

section containing vegetation within the 1 mil acre plot was recorded. Using the density

comparison gauge, scores for density were recorded as follows: 0 = no section with vegetation, 1

= 1 – 3 vegetated sections, 2 = 4 – 6 vegetated sections and 3 = 7 – 9 vegetated sections. Density

readings were taken at cardinal headings and averaged for each plot. Vegetation Density is based

on a combination of average percent cover or all vegetation types and the average height of

vegetation at 1 meter and categorized as: Sparse Vegetation ( ≥ 60% bare ground with limited

shrub, grass, and forb cover that seldom exceed 1 m in height); Dense Short Vegetation (60 -

80% shrub cover interspersed with grasses, forbs, and shrubs all occasionally exceeding 1 meter

in height with ≤ 40% bare ground and some small saplings < 3 m tall); Very Dense Vegetation (>

80% shrub cover with most grasses, forbs, shrubs, and saplings exceeding 1 meter in height, with

≤ 20% bare ground and some small trees > 3m tall); and All Vegetation Very Dense & Tall (>

11

80% shrub cover with all vegetation types averaging > 1m tall, some trees > 3m tall , and

minimal to no bare ground).

2.3.2. Landscape Features

To estimate the landscape composition surrounding each shrubland, six buffer distances (50

m, 250 m, 500 m, 1 km, 5 km, and 10 km) were establish around each shrubland using ArcGIS

10.2.2. The 28 LANDFIRE 2014 Existing Vegetation Height (30 m pixels) (LANDFIRE 2014)

data land categories were combined into seven surrounding landscape features and then

estimated within each buffer distance (Appendix B). Specifically, I estimated the proportion of

the landscape that was Agriculture, Field/Pasture, Open Water, Shrubland, Tidal Vegetation,

Forest, and Urban Development within each buffer. Although the larger buffer landscape

features incorporated all subsequent smaller buffer landscape features, all surrounding landscape

features within each buffer were retained in the analyses regardless of correlation strength due to

buffer overlap. Variability in the percentages of surrounding landscape features decreases as

buffer distances decrease around the shrublands.

I was unable to examine all vegetation types in every buffer distance and categorizing

LANDFIRE data into surrounding landscape features had its challenges. Growth of vegetation

over time, misinterpreted vegetation types within LANDFIRE, and the combining of

LANDFIRE into only seven types of surrounding landscape features may affect the responses of

the shrubland birds. I found most representations of LANDFIRE data did match the vegetation at

the surveyed sites after two years or was within the appropriate categories I selected as

surrounding landscape features.

12

TABLE 1. LANDFIRE 2014 Existing Vegetation Height (30 m pixels) categories grouped into

seven surrounding landscape features with reasons for grouping.

LANDFIRE 2014 category Surrounding landscape

feature category

Reason

Barren Shrubland Sparse shrub vegetation

Developed-Roads Urban Development Urban development

Developed-Upland Deciduous Forest Trees Trees in developed areas

Developed-Upland Evergreen Forest Trees Trees in rural and developed areas

Developed-Upland Herbaceous Fields, Pastures Golf courses/fields: mowed often

Developed-Upland Mixed Forest Trees Tree pockets in developed areas

Developed-Upland Shrubland Urban Development Small backyards by rural roads

Developed-High Intensity Urban Development Urban development

Developed-Medium Intensity Urban Development Urban development

Developed-Low Intensity Urban Development Urban development

Forest Height 0 to 5 meters Forest Forest

Forest Height 5 to 10 meters Forest Forest

Forest Height 10 to 25 meters Forest Forest

Forest Height 25 to 50 meters Forest Forest

Herb Height 0 to 0.5 meters Tidal Areas next to rivers, ponds, lakes

Herb Height 0.5 to 1.0 meters Fields, Pastures Most are fields, and pastures

Herb Height > 1.0 meters Fields, Pastures Most are fields, and pastures

NASS-Close Grown Crop Agriculture Agriculture

NASS-Row Crop Agriculture Agriculture

NASS-Row Crop-Close Grown Crop Agriculture Agriculture

NASS-Vineyard Agriculture Agriculture

NASS-Wheat Agriculture Agriculture

Open Water Open water Open water

Quarries-Strip Mines-Gravel Pits Shrubland Sparse shrub vegetation

Shrub Height 0 to 0.5 meters Shrubland Shrubland

Shrub Height 0.5 to 1.0 meters Shrubland Shrubland

Shrub Height > 3.0 meters Shrubland Shrubland

Sparse Vegetation Height Shrubland Shrubland

13

2.4. Statistical Analysis

2.4.1. Bird Occurrence

Differences in detection likelihood among shrubland types were analyzed in JMP Pro

13.0.0 (SAS Institute, Cary, NC) for each species using Kruskal-Wallis followed by Wilcoxon

tests among all pairs with a test for false discovery rate for significance.

2.4.2 Site-Specific and Landscape Features Affecting Occurrence

Shrubland bird associations between the species occurrence and local and landscape features

were analyzed using non-metric multidimensional scaling (NMDS) in PC-ORD v.6.19 (MjM

Software Gleneden Beach, OR). Ordination was performed with the Sorensen distance measure

in the Autopilot Slow and Thorough mode (McCune and Grace 2002), with 250 runs using both

real and randomized data. NMDS supports presence/absence, categorical, and continuous data

allowing for unconstrained analysis. The secondary matrix of shrubland and surrounding

landscape features was generally relativized to sum of squares (P = 1) to standardize to the norm

for Sorensen distance measure. In NMDS, goodness-of-fit is measured by a stress value used to

determine the number of dimensions that adequately represent sample units in ordination space

and to indicate how well the configuration matches the data (Kruskal 1964). The starting

configuration was optimized in previous ordinations to achieve the lowest final stress (8.813) and

both 2 and 3-dimensionalities were assessed. I selected the programs default joint plot cutoff (R2

= 0.20) representing the minimum value identifying a strong relationship between the

environmental variables and shrubland bird occurrences.

Focal species likelihood of occurrence in shrublands was assessed with nonparametric

multiplicative regression (NPMR) using Hyperniche v.2.30 (MjM Software, Gleneden Beach,

OR) because I expected birds would have a complex, non-linear or even an asymmetrical

14

response to variables. NPMR creates a multidimensional environmental space unconstrained by

linear responses and uses multiplicity of the variables to model species responses (McCune and

Mefford 2009). Specifically, it uses all the variables, and combinations of all variables, to target

the best location in multidimensional space identifying successively important variables that best

explain the likelihood of a species occurring (McCune and Mefford 2009). Shrubland types were

combined to determine focal species occurrence associated with surrounding landscape features.

Model form was set to Local Mean – Gaussian to center the probability density function on the

target point achieving full weighting for an observation with the same environment as the target

point. This allows a diminishing weight in observations with increasing distance from the target

point. All other settings for Free Search were set to defaults. I assessed model quality by using

the maximum cross-validated coefficient of determination R2 (xR2) from the best fit models

(McCune and Mefford 2009) and conducted sensitivity analyses. Sensitivity is expressed as a

proportion of the range of the response variable within each model (values are only compared to

each other within the model) and higher values have more influence and indicate the species is

more sensitive to the model (McCune and Mefford 2009). The minimum average neighborhood

size for acceptable model, and minimum neighborhood size for estimate, were set at automatic

and the response curves overfitting controls were set at medium. In the predicted occurrence

percent change, I considered a response to be negligible if the response achieved < 20% change

over the range of the variable.

To determine whether a species responded more to site-specific or surrounding landscape

features I totaled the number of features comprising each species’ best NPMR model and

calculated a percentage of the total composed by each category of features to determine which

was greater.

15

3. RESULTS

3.1. Shrubland Bird Detection and Shrubland Types

All but one of the eight focal species were detected at least once in every shrubland type;

brown thrashers were detected in all shrubland types except clearcuts (Fig 2). P-values are

derived from Kruskal-Wallis followed by Wilcoxon tests among all pairs with a test for false

discovery rate for significance. Brown thrashers were more likely (P < 0.008) to be detected in

sand and gravel mines than in the other shrubland types. Prairie warblers and field sparrows were

more likely (P < 0.009 for both) to be detected in ROWs and sand and gravel mines than in

clearcuts or old fields. Eastern towhee detections were greatest in ROWs and sand and gravel

mines, and the proportion of detections in ROWs was greater (P < 0.004) than that in clearcuts

and old fields. Chestnut-sided warbler detections were greatest in ROWs and clearcuts, and the

proportion of detections in ROWs was greater (P < 0.006) than those in old fields and sand and

gravel mines. Indigo bunting detections were greatest in sand and gravel mines and ROWs; the

proportion of indigo bunting detections in sand and gravel mines was greater (P < 0.002) than

those in clearcuts and those in old fields (P < 0.007); those in ROWs were greater (P < 0.019)

than in clearcuts. There was no difference in detections among shrubland types for alder

flycatchers or blue-winged warblers.

16

FIGURE 2. Comparing the likelihood that each of eight focal shrubland bird species was

detected within each of four anthropogenic shrubland types occurring in Rockingham and

Strafford Counties, New Hampshire May-August 2015 and 2016. Differences in detections

analyzed using Kruskal-Wallis followed by Wilcoxon tests among pairs with a false discovery

rate for significance. Bars with different letters differ (P < 0.05).

3.2. Bird Responses to Site-Specific and Landscape Features

Site-specific features for shrublands combined had averages for Size (9.7 ha), Perimeter (1.9

km), Vegetation Density (1.5), Species Richness (4.8), Open Water cover (0.2%), Bare Ground

cover (35.6%), Grass cover (34%), Fern cover (9.4%), Native Shrub cover (33.3%), Non-native

Shrub cover (7%), Tree cover (7.4%) and Forb cover (33.1%). All shrubland site-specific

features with ranges and averages for sand and gravel mines (S&GM), transmission line rights-

of-way (ROW), old fields (OF), and clearcuts (CC) are in Appendix A.

Overall, Urban development and Field/Pasture increase in the surrounding landscape out to

500 meters and then decrease from 1 km to 10 km. Surrounding Shrubland in the landscape

decreases as distance increases from the shrubland and there is less than a 3% change of Forest

17

in the surrounding landscape at all buffer distances (Appendix B). All shrubland surrounding

landscape feature ranges and averages for sand and gravel mines (S&GM), transmission line

rights-of-way (ROW), old fields (OF), and clearcuts (CC) are in Appendix B.

The best-fit NPMR models included site-specific and landscape features (Fig 3). Three birds

(alder flycatcher, brown thrasher, chestnut-sided warbler) were positively associated with

surrounding Shrubland. Five birds (brown thrasher, prairie warbler, eastern towhee, alder

flycatcher, field sparrow) were negatively associated with surrounding Field/Pasture and the

blue-winged warbler was positively associated with Field/Pasture. Two birds (chestnut-sided

warbler, indigo bunting) were negatively associated with surrounding Urban Development. Two

birds (field sparrow, blue-winged warbler) were positively associated with Tidal Vegetation and

two birds (indigo bunting, prairie warbler) were negatively associated with Tidal Vegetation.

Two birds (brown thrasher, field sparrow) were negatively associated with Open Water and the

prairie warbler was positively associated with Open Water. The alder flycatcher was positively

associated with Agriculture and the blue-winged warbler was negatively associated with

Agriculture. Two birds (indigo bunting, chestnut-sided warbler) were positively associated with

Vegetation Density and the brown thrasher was negatively associated with Vegetation Density.

Two birds (eastern towhee, field sparrow) were associated positively with Shrubland Size. The

blue-winged warbler was positively associated with Non-native shrubs. The alder flycatcher was

negatively associated with Bare Ground Cover and the eastern towhee was positively associated

with Bare Ground Cover. The chestnut-sided warbler was negatively associated with Tree

Cover. The prairie warbler was negatively associated with Grass Cover.

18

FIGURE 3. Best NPMR models predicting focal bird occurrence and bird sensitivity to an

increasing proportion of site-specific and surrounding landscape features. Positive (+) and

negative (-) signs indicate the direction of association of the species to the variable. Higher

sensitivity values indicate the predicted occurrence of the species changes readily

(positive/negative) in response to changes in the variable.

19

The importance of site-specific and surrounding landscape features as predictors of focal bird

occurrence differed among the species (Fig 4). Specifically, surrounding landscape features

comprised 75% of the feature types associated with alder flycatchers, brown thrashers, blue-

winged warblers, field sparrows, and prairie warblers and 67% of the feature types associated

with indigo buntings. Comparatively, the site-specific features comprised 67% of the feature

types associated with eastern towhees. The site-specific and surrounding landscape features each

comprised 50% of the feature types associated with chestnut-sided warblers (Fig 4). These

results were derived from the best models that explained the occurrences of each focal species

(Fig 3).

FIGURE 4. The proportion of the site-specific features (SSF) and surrounding landscape

features (SLF) comprising the best fit NPMR models predicting focal species occurrence in 101

anthropogenic shrublands in southeastern New Hampshire 2015-2016.

Combined, Shrubland, Field/Pasture, and Urban Development account for 55% of

surrounding landscape features identified in the best fit models predicting the occurrence for all

species. The average percent change in predicted occurrence of all species was associated

positively with increasing Shrubland and variable, by species, with increasing Field/Pasture and

20

Urban Development across all buffer distances (Fig 5). Specifically, alder flycatchers, blue-

winged warblers and chestnut-sided warblers responded positively, and all other species

responded negatively, to Field/Pasture. Blue-winged warblers and indigo buntings responded

positively, and all other species responded negatively, to Urban Development.

FIGURE 5. Average percent change in predicted occurrence for eight focal shrubland bird

species in response to increasing proportions of Shrubland, Field/Pasture, and Urban

Development in the surrounding landscape (50 m – 10 km) from the best fit NPMR models.

Positive value indicates an increase in average predicted occurrence and negative value indicates

a decrease in average predicted occurrence.

A convergent three-dimensional solution was reached by NMDS comparing site-specific and

surrounding landscape features to shrubland bird occurrence (Fig 6). The three axes captured a

total of 91.8% of the variance in the model (axis 1: 44.8%, axis 2: 33.4%, axis 3: 13.5%). Axes 1

and 2 had the highest coefficients of determination and explained 78.2% of the shrubland bird

occurrence in relation to the site-specific and surrounding landscape feature data. Species

richness (R2 = 0.320) and Field/Pasture within 1 km (R2 = 0.211) met the minimum association

of R2 ≥ 0.2 and had the strongest relationship between the site-specific and surrounding

21

landscape features to the ordination scores in ordination space. Specifically, the chestnut-sided

warblers and blue-winged warblers had positive associations with Field/Pasture within 1 km as

it increased from axis 1, and Species richness increased as Vegetation density increased (Fig 6).

FIGURE 6. Nonmetric multidimensional scaling (NMDS) of eight focal shrubland bird species

in a community structure of 101 anthropogenic shrublands in southeastern New Hampshire,

USA, 2015-2016. Field/Pasture within 1 km and Species richness vectors are oriented toward the

direction of greatest increase and vector lengths are proportional to R2 with the ordination. The

angle between vectors indicates the correlation between variables. Convex hulls indicate

Vegetation Density categories and are defined in METHODS 2.3.1.

22

Based on the related strength of Species Richness to Vegetation Density a separate analysis

was conducted comparing Species Richness to Vegetation Density. All eight focal species were

associated with Dense Short Vegetation (n = 27) and Very Dense Vegetation (n = 38), which,

combined had the most shrublands associated with them. Species Richness in Sparse Vegetation

Density (n = 22) was limited to seven of the focal species with blue-winged warbler absent and

Species Richness in All Vegetation Very Dense and Tall (n = 14) was limited to five species with

the brown thrasher, prairie warbler and field sparrow absent.

The NPMR indicated either a positive or negative percent change in predicted occurrence to

increasing Vegetation Density (Fig 7). The alder flycatchers, blue-winged warblers, indigo

buntings, chestnut-sided warblers and eastern towhees had an overall positive predicted

occurrence as Vegetation Density increased. The brown thrashers, field sparrows, and prairie

warblers had and overall negative predicted occurrence as Vegetation Density increased.

23

FIGURE 7. NPMR comparing predicted occurrence of eight focal shrubland bird species in

response to increasing Vegetation Density estimated in 101 anthropogenic shrublands in

Strafford and Rockingham Counties New Hampshire 2015-2016. Percent increase/decrease in

predicted occurrence is indicated in parentheses. Alder flycatchers, brown thrashers, blue-winged

warblers, chestnut-sided warblers, indigo buntings, and prairie warblers have significant (>20%

change) predicted occurrence responses.

3.3. Bird Responses to Shrubland Size

The NPMR model indicated either a positive or negative percent change (% change) in

predicted occurrence of the eight focal shrubland bird species in response to increasing

Shrubland Size and the association was generally positive (Fig 8). For alder flycatchers (%

change = 43.0), brown thrashers (% change = 44.2), eastern towhees (% change = 31.1), field

sparrows (% change = 53.1), prairie warblers (% change = 27.4), and indigo buntings (% change

= 9.2) the predicted percent change in response to increasing Shrubland Size was clearly and

consistently positive. Blue-winged warblers (% change = -4.3) had a minimal negative response

to increasing Shrubland Size. Chestnut-sided warblers (% change = 1.1) had a positive response

to increasing Shrubland Size that peaked around 7.8 ha in Shrubland Size, then the response was

negative as Shrubland Size increased. In the NPMR model (Fig 8), the response lines for the

indigo buntings and brown thrashers are longer because a greater number of predicted

24

occurrence responses in sand and gravel mines met the minimum neighborhood size for

inclusion in the analysis.

FIGURE 8. NPMR comparing predicted occurrence of eight focal shrubland bird species in

response to increasing Shrubland Size (ha) estimated in 101 anthropogenic shrublands in

Strafford and Rockingham Counties New Hampshire 2015-2016. Percent increase/decrease in

predicted occurrence is indicated in parentheses.

4. DISCUSSION

Contrary to previous studies (Bulluck and Buehler 2006, Chandler 2006, Shake et al. 2012), I

found that shrubland bird occurrence was better predicted by the composition of the surrounding

landscape than by habitat features measured within shrublands. But similar to those and other

studies (King et al. 2009b, Bauer 2018), shrubland bird occurrence and species richness were

associated positively with vegetation density within shrublands and shrubland size. Specifically,

shrubland birds as a group were more commonly present in shrublands situated in landscapes

where the surrounding 500 m was composed of a greater proportion of shrublands than of fields

or urban development, in shrublands > 12 ha, and in shrublands with Dense Short Vegetation or

25

Very Dense Vegetation. These results can serve as a basic guide for identifying, managing and

conserving shrublands that are likely to support the greatest diversity of shrubland birds within

contemporary landscapes. However, species-specific preferences for specific microhabitats (e.g.,

sparse/dense/short/tall vegetation, amount of bare ground, mesic/xeric soils) indicates each bird

species will occur most predictability in shrublands that support its preferred microhabitats, and

local landscapes composed of multiple shrubland types (e.g., ROW, clearcuts, old fields, sand

and gravel mines, pine barrens, beaver meadows) occurring across a range of growing conditions

(e.g., mesic, xeric), should support the greatest diversity and abundance of shrubland birds

(Chandler et al. 2009b, King et al. 2009a, Gifford et al. 2010). Overall, sand and gravel mines

and ROW were the largest shrublands in my study and they supported both the greatest diversity

of microhabitats and the greatest species richness of shrubland birds (Fig 2).

4.1. Importance of Landscape Composition to Breeding Shrubland Birds

Occurrence of all species was associated positively with increasing proportions of Shrublands

in the surrounding landscape. All species except the blue-winged warbler, chestnut-sided

warbler, and alder flycatcher were associated negatively with increasing proportions of

Field/Pasture and the indigo bunting and blue-winged warbler were associated positively to

increasing proportions of Urban Development in the surrounding landscape (Fig 5). The positive

response I observed for blue-winged warblers to Urban Development and Fields/Pasture (Fig 5)

is likely because these land uses were more common within 500 m around old fields, where blue-

winged warbler occurrence was greatest (Fig 2), than around the other shrubland types in my

study area. The negative response and sensitively of most focal birds to greater proportions of

Field/Pasture and Urban Development may be associated with attributes of these habitats that

can have negative consequences to breeding bird fitness. For example, fragmentation by human

26

development and agriculture can decrease nesting success due to increased nest predation

(Rudnicky and Hunter 1993, Rodewald and Yahner 2001) and/or by parasitism by brown-headed

cowbirds (Burhans and Thompson III 2006, Askins et al. 2012). Urban Development may further

reduce nesting success in shrublands due to elevated noise and light levels (Oneal and

Rotenberry 2009) and/or reduced food and cover resources compared to when the adjacent

habitat is mature forest (King and DeGraaf 2004, Oneal and Rotenberry 2009, Shake et al. 2012).

Shrubland bird fitness should be greatest in landscapes where multiple shrublands are

clustered in close proximity within a matrix composed primarily of forest and wetlands and a

low proportion of fields, pasture, agricultural crops or urban development (Roberts and King

2017). Specifically, shrubland birds are known to increase their fitness by readily moving among

adjacent habitat patches during a breeding season to seek extra-pair copulations (Nolan 1978,

Byers et al. 2004, Akresh et al. 2015), to relocate following nest failure (Best 1977, Nolan 1978),

and possibly to scout and assess other potential habitats (Nolan 1978, Whitaker and Warkentin

2010). Also, this large-scale habitat selection may also indicate shrubland birds may have more

movement during the breeding season (Lehnen and Rodewald 2009b) than previously examined.

However, the ideal distance shrublands must be from one another to facilitate these bird

movements requires further study. Roberts and King (2017) determined that small (0.02 -1.3 ha)

shrublands were more likely to be occupied by shrubland birds when those shrublands occurred

within < 100 m of larger shrublands, and Lehnen and Rodewald (2009a) recommended

clustering shrublands within 1 km of one another to facilitate bird movements.

During my study, I observed individual color-banded prairie warblers, field sparrows, and

eastern towhees movements averaging 6.7 km, and as much as 29 km, within multiple

shrublands during the same nesting season (R. Shoe, University of New Hampshire, unpublished

27

data). Although these birds used multiple shrublands juxtaposed across a large local landscape, I

found shrubland bird occurrence was best predicted by the proportion of shrublands within 500

m of my study sites, likely because the proportion of shrublands in the landscape around my sites

decreased as distance from sites increased. It is possible that shrubland bird occurrence within

500 m of my sites may be due to neighborhood fidelity, whereas a bird has fidelity to multiple

sites within a radius that supports its habitat preference. More study is needed on the possible

neighborhood fidelity of shrubland birds. My results suggest clustering shrublands within 500 m

of one another may be best for encouraging shrubland bird occurrence in anthropogenic

shrublands within a suburban landscape and may possibly support neighborhood fidelity.

Unlike the other focal species, occurrence of chestnut-sided warblers was predicted equally

by site-specific and surrounding landscape features (Fig 4). The chestnut-sided warbler is

strongly influenced by microhabitat and landscape-level variables (Hagan and Meehan 2002,

Chandler 2006) and its positive response to Vegetation Density and negative response to Tree

Cover (Fig 3) are consistent with other studies (Askins et al. 2007, 2012). Movement by

chestnut-sided warblers during the breeding season can be limited by Urban Development (Byers

et al. 2004), and this may explain the negative predicted occurrence I observed for chestnut-sided

warblers in response to Urban Development within 50 m (Fig 3). An explanation of why the

chestnut-sided warbler had a positive response to a decreasing proportion of Shrublands out to

10 km (Fig 3) may be as the amount of surrounding Shrublands decrease from the occupied

habitat and the amount of Forest increases, it may be chestnut-sided warblers are responding to

its preference of an increasing proportion of understory stems < 1m tall (Hagan and Meehan

2002), provided that the forest has limited canopy cover and sufficient understory. Similar to my

other focal species, shrublands located in a landscape with minimum Urban Development are

28

those most likely to benefit chestnut-sided warblers. Managers can further attract chestnut-sided

warblers to shrublands by encouraging microhabitats composed of dense to very dense

vegetation surrounded by forest with an understory growth.

The eastern towhee was the only focal species for which site-specific features better

predicted occurrence than the surrounding landscape features (Fig 4) and towhees were most

sensitive (positively) to Bare Ground Cover and Shrubland Size (Fig 3). Increased sensitivity to

Bare Ground Cover may be explained by the habit of eastern towhees to forage and nest on the

ground in habitats with leaf litter and dense grasses, forbs and ferns. Towhees are reported to use

shrublands that span a range of sizes (0.02 - 21 ha: Askins et al. 2007, Roberts and King 2017),

and in my study area, they seem to use shrubland openings of any size as long as the shrubland

supports their foraging and nesting habits. Towhee sensitivity to Shrubland size is likely because

they occurred most predictably in large ROW and sand and gravel mines; these large openings

supported a variety of microhabitat conditions, including areas of Bare Ground Cover, Dense

Short Shrubs, and Very Dense Shrubs that satisfy multiple towhee habitat preferences. Managers

can make shrublands more suitable for eastern towhees by ensuring a site has dense to very

dense vegetation for nesting and bare ground with leaf litter for foraging.

4.2. Vegetation Structure, not Species Composition, Predicted Shrubland Bird Occurrence

Vegetation Density within shrublands had an overall positive influence on focal bird richness

and shrublands composed of Dense Short Vegetation and Very Dense Vegetation were those

most likely to be occupied by all eight focal species. However, bird response to Vegetation

Density was species-specific. As expected, brown thrashers, prairie warblers and field sparrows

were most likely to occur in shrublands with Sparse, Dense Short, and Very Dense Vegetation

and they were least likely to occur in shrublands with All Vegetation Very Dense and Tall.

29

Comparatively, blue-winged warblers and alder flycatchers were more likely to occur in

shrublands with All Vegetation Very Dense & Tall, and they were less common in shrublands

composed of Sparse Vegetation or Dense Short Vegetation. Chestnut-sided warblers were most

predictable in shrublands composed of Dense Short Vegetation or Very Dense Vegetation and

they were least likely to occur in All Vegetation Very Dense & Tall shrublands. Indigo buntings

and eastern towhees were present in all shrublands sampled and among all species, their

occurrence seemed the least influenced by vegetation density.

Because bird responses to vegetation density was species-specific, large shrublands composed

of a variety of microhabitats that support vegetation conditions ranging from Sparse Shrub

Vegetation to All Vegetation Very Dense & Tall are those shrublands most likely to support the

greatest shrubland bird richness. In my study, sand and gravel mines and ROW’s were the largest

shrublands I surveyed, and they supported both the greatest diversity of microhabitats and the

greatest shrubland bird richness (5.4 and 5.2 respectively). Specifically, sand and gravel mines

contained microhabitats consisting of mesic and xeric areas, low-growing and tall-growing

shrubs, and extensive bare ground, and they were also the shrublands with the greatest proportion

of other shrublands in the surrounding landscape; combined, these characteristics of sand and

gravel mines likely explain why they had the highest species richness of all shrubland types

surveyed. In some landscapes, sand and gravel mines may functions as the most stable and

predictable habitats for shrubland birds due to their harsh growing conditions that naturally keep

them in a shrubby condition longer than other anthropogenic shrublands such as clearcuts and

old fields where shrubby conditions are more ephemeral (Borgegard 1990, Bulluck and Buehler

2006). Regular maintenance of ROW by utility companies creates an additional source of diverse

and predictable shrubland habitat supporting a variety of shrubland bird species (King and Byers

30

2002, King et al. 2009a, Askins et al. 2012, this study) resulting from the diverse vegetation

conditions within the length of most ROW. My study indicates sand and gravel mines and

transmission ROW should be considered important habitats that can contribute to the long-term

conservation of declining shrubland birds in suburban New England landscapes.

I suspect the type of shrubland may also affect species richness with vegetation density,

especially in sand and gravel mines and ROW. Although sand and gravel mines have the highest

species richness and four of the highest detections for the species, I believe their size and the

features in the surrounding landscape may be what affects species richness more than vegetation

density. Sand and gravel mines, with less shrubs than old fields, clearcuts and ROW, have the

most shrublands surrounding them out to 500 m more than any other shrubland type. More

shrublands surround sand and gravel mines than any other shrubland type, and the value of

shrublands is important to all the species studied (Fig 5). Active sand and gravel mines and

ROW, ranging from Sparse to Very Dense Vegetation, are the only two anthropogenic habitats I

studied maintained in a shrubby condition. Habitats maintained in a shrubby condition have

indicated over a three-year mark and recapture study (2014 – 2016) that 85% of the yearly re-

sighted or re-captured prairie warblers, field sparrow, and eastern towhees have shown site

fidelity to active sand and gravel mines and ROW (R. Shoe, University of New Hampshire,

unpublished data). ROW’s are also well connected and have varying mowing schedules and

likely offer safer or easier species movement and habitat selection of desirable vegetation

densities, thereby increasing species richness.

Managers can attempt to make each shrubland suitable for a variety of shrubland bird species

by managing for a diversity of vegetation densities that are most suited for the growing

conditions of each shrubland. It may not be possible or feasible to increase vegetation diversity

31

in a shrubland composed of homogenous growing conditions; here, the best option for increasing

vegetation diversity is to identify opportunities within the surrounding 500 m landscape where

additional shrublands can be created or maintained on sites that can support different and

complimentary vegetation conditions.

Except for blue-winged warblers, vegetation species composition did not predict the

occurrence of focal shrubland birds (Fig 3). In my study area, shrubland bird richness has been

shown to increase as the proportion of invasive shrubs (e.g., Frangula alnus, Eleagnus

umbellata, Lonicera spp.) in a shrubland increases from zero to about 55% of the total shrub

cover (Tarr 2017, Bauer 2018). My study did not indicate this response for the shrubland bird

species, likely because I measured the percent cover of native and invasive shrubs as a

proportion of total vegetation cover (including grasses, forbs, ferns, trees, bare ground, and open

water), and both Bauer (2018) and Tarr (2017) measured native/invasive shrub cover as a

proportion of total shrub cover only. As a result, my sites averaged 7% Non-native shrub cover

and 33% Native shrub cover which may explain why Species Richness increased with vegetation

structure and not vegetation species composition. Blue-winged warblers responded positively to

increasing proportions of Non-native shrub cover (Fig 3), but because non-native shrubs were

most abundant in old fields, the shrubland type where blue-winged warblers were most

predictable, I cannot conclude whether they were responding specifically to Non-native shrub

cover or the high vegetation densities of old fields, coinciding with King et al. (2009b). Overall,

my results indicate that vegetation density and height was more important for influencing

shrubland bird distribution than was the native/non-native shrub composition in shrublands.

32

4.3. Shrubland Birds Occur More Predictably in Larger Shrublands

Most bird species occurred with increasing predictability as shrubland size increased above

0.8 ha (Fig 8). I predicted bird occurrence would increase with shrubland size because larger

shrublands should provide birds with a lower risk of edge effects such as nest predation (King

and Byers 2002, DeGraaf and Yamasaki 2003), and compared to smaller shrublands, they are

more likely to support a greater variety of microhabitats that support shrubland birds (Rodewald

and Vitz 2005, Chandler et al. 2009b, Singer et al. 2012). Chestnut-sided warblers and blue-

winged warblers were the exceptions to the pattern of greater predictability in response to larger

shrubland size (Fig 8). Chestnut-sided warbler occurrence became less predictable when

shrubland size exceeded 7.8 ha (Fig 8), likely explained by the fact that most shrublands larger

than 7.8 ha in my study were sand and gravel mines where the microhabitats preferred by

chestnut sided-warblers were uncommon. Similarly, blue-winged warbler occurrence was largely

unresponsive to differences in shrubland size, likely because they occurred most predictably in

old fields (Fig 2) characterized by All Vegetation Very Dense and Tall, a microhabitat type that

was uncommon in the larger ROW and sand and gravel mines in my study. Although, by not

limiting the shrubland size during my study to a maximum value, the sand and gravel mines,

averaging 17.3 ha may have swayed the predicted occurrence of the blue-winged warbler and

chestnut-sided warbler, since the sand and gravel mines were significantly larger than all other

shrubland types except ROW’s. Managing for large shrublands has long been a conservation

strategy to increase shrubland bird species richness (DeGraaf and Yamasaki 2003, Shake et al.

2012, Roberts and King 2017) and my findings agree with this strategy.

33

5. CONCLUSION AND MANAGEMENT RECOMMENDATIONS

My study was done in the southeastern seacoast of New Hampshire and my results may not

be applicable to northern or southern areas based on forest types, soil composition and because

this area is the northernmost range of the blue-winged warblers, eastern towhees, and prairie

warblers (Dunn and Alderfer 2011). The presence and absence of the blue-winged warblers,

eastern towhees, and prairie warblers may have been affected by their availability earlier or later

in the breeding season in this northernmost range of their habitat. No shrubland bird in my study

had a modeled response to Forest in the surrounding landscape and only the chestnut-sided

warbler had a response to site-specific Tree cover (negative). I suspect in heavily forested areas

the shrubland bird occurrences would be less, as found in other studies (Costello et al. 2000,

Chandler 2006), than the shrubland bird occurrences in the seacoast of New Hampshire, possibly

based on the extent of the surrounding forest, size and type of opening, and a lack of alternate

shrublands within the surrounding landscape.

Increased shrubland size and increasing vegetation density should be considered during the

planning stages of conservation site selection for shrubland birds, as most of the shrubland birds

responded positively to an increase in Size and Vegetation Density. Because shrubland Size did

not correlate to Vegetation Density, the predicted occurrence of the shrubland birds to shrubland

Size are based on the species habitat preferences to other site-specific and surrounding landscape

features. Most species predicted occurrence were in response to surrounding landscape features

(Fig 4) yet evaluating an appropriate size for shrubland bird conservation based on surrounding

landscape features of up to 10 km is unreasonable for most applications. Focusing on the site-

specific features (Size and Vegetation Density) and adjacent surrounding landscape features

(Shrublands, Urban Development, Field/Pasture) that best predicted most species occurrences

34

may be best when making a conservation decision regarding shrubland birds. Also, the trend of

increasing Urban Development and Field/Pastures as Shrubland decreases in the adjacent

surrounding landscape needs to be examined more closely. By combining the site-specific and

adjacent surrounding landscape features the birds are responding to, managers may be able to

determine if the size of a shrubland can be ‘extended’ with limited consideration to local

surrounding landscape features to increase the shrubland.

More study is also needed for shrubland bird conservation within sand and gravel mines

and ROW’s which seem to have high value to shrubland birds. Incorporating anthropogenic

shrublands maintained in a shrubby condition, such as ROW, or slow growing for an extended

time, such as sand and gravel mines (Rehounková and Prach 2006), should be considered as

primary conservation areas for shrubland birds. Generally, increasing shrub (native or exotic)

density within the shrubland habitat increases the predicted occupancy of most of the shrubland

bird species we studied.

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41

APPENDIX A

Shrubland site-specific features for sand and gravel mines (S&GM), transmission line rights-of-

way (ROW), old fields (OF), and clearcuts (CC). Size range (ha) and Species Richness are

included and are not considered site-specific features.

Variable Range S&GM

Average

ROW

Average

OF

Average

CC

Average

Overall

Average

Size (ha) 1-91 17.3 7.77 6.87 6.66 9.7

Size range (ha) 2-91 1-26 1-22 1-24 9.7

Perimeter (km) 0.4-12 2.2 3.1 1.4 1.3 1.9

Vegetation density 0-3 0.82 1.78 1.79 1.54 1.5

Open Water cover (%) 0-4 0 0.3 0.3 0.2 0.2

Bare Ground cover (%) 2-85 59.6 26.6 20.9 35.3 35.6

Grass cover (%) 1-91 24.2 25.6 55.7 30.5 34

Fern cover (%) 0-48 1.2 20.2 7.2 8.8 9.4

Native Shrub cover (%) 1-82 23.8 45 27.2 37.3 33.3

Non-native shrub cover (%) 0-43 2.4 8.7 8.2 8.7 7

Tree cover (%) 0-38 7.5 4.4 5.7 11.9 7.4

Forb cover (%) 1-68 25.8 30.1 44.5 32 33.1

Species Richness 1-8 5.4 5.2 4.4 4.1 4.8

42

APPENDIX B

Shrubland surrounding landscape features, ranges and averages for sand and gravel mines

(S&GM), transmission line rights-of-way (ROW), old fields (OF), and clearcuts (CC).

Variable Range

(%)

S&GM

Averages

(%)

ROW

Averages

(%)

OF

Averages

(%)

CC

Averages

(%)

Overall

Average

(%)

50m agriculture 0-1 0.00 0.11 0.06 0.00 0.04

50m field/pasture 0-62 3.70 7.06 17.86 4.24 8.21

50m open water 0-4 0.26 0.20 0.10 0.06 0.16

50m shrubs 0-71 28.79 8.02 12.77 4.68 13.57

50m tidal vegetation 0-5 0.07 0.10 0.07 0.49 0.18

50m forest 9-100 55.82 66.46 47.08 81.63 62.75

50m urban development 0-72 11.38 18.04 22.06 8.90 15.09

250m agriculture 0-1 0.01 0.13 0.05 0.02 0.05

250m field/pasture 0-44 6.14 8.04 14.33 8.98 9.37

250m open water 0-13 0.57 1.07 0.76 0.20 0.65

250m shrubs 0-38 15.84 4.68 7.44 6.59 8.64

250m tidal vegetation 0-2 0.12 0.11 0.08 0.16 0.12

250m forest 6-99 65.21 63.17 52.72 70.29 62.85

250m urban development 0-81 12.11 22.79 24.63 13.76 18.32

500m agriculture 0-3 0.01 0.13 0.25 0.03 0.11

500m field/pasture 1-37 6.03 8.85 11.86 12.06 9.70

500m open water 0-20 0.36 1.66 1.17 0.52 0.93

500m shrubs 0-33 11.14 4.58 6.08 6.17 6.99

500m tidal vegetation 0-1 0.10 0.06 0.06 0.13 0.09

500m forest 10-92 68.12 59.78 54.78 65.04 61.93

500m urban development 0-73 14.23 24.93 25.80 16.05 20.25

1km agriculture 0-4 0.13 0.10 0.34 0.13 0.17

1km field/pasture 1-36 6.92 9.66 10.52 12.87 9.99

1km open water 0-20 0.42 2.34 1.07 1.41 1.31

1km shrubs 1-20 8.85 5.15 5.57 5.43 6.25

1km tidal vegetation 0-2 0.25 0.13 0.06 0.16 0.15

1km forest 10-91 70.29 57.19 57.01 63.61 62.03

1km urban development 3-77 13.15 25.43 25.44 16.38 20.10

5km agriculture 0 0.14 0.15 0.22 0.20 0.18

5km field/pasture 3-15 7.66 9.05 11.08 10.12 9.48

5km open water 0-18 2.22 2.90 2.33 3.47 2.73

5km shrubs 3-8 5.70 5.18 5.73 5.20 5.45

5km tidal vegetation 0-1 0.16 0.20 0.17 0.22 0.19

5km forest 32-89 71.27 61.88 58.44 62.92 63.63

5km urban development 4-42 12.85 20.64 22.03 17.87 18.35

10km agriculture 0 0.16 0.16 0.18 0.18 0.17

10km field/pasture 3-13 7.30 8.48 10.18 9.64 8.90

10km open water 1-35 3.92 3.89 4.39 5.27 4.37

10km shrubs 3-6 5.01 5.02 5.35 5.16 5.13

10km tidal vegetation 0-1 0.19 0.24 0.23 0.27 0.23

10km forest 32-85 70.57 67.81 62.27 64.58 66.30

10km urban development 5-21 12.86 14.41 17.39 14.90 14.89

43

APPENDIX C